The present invention relates to a method for extracting fossil fuels, and more particularly to a method for recovering gas from a gas hydrate deposited in a formation underground or under the sea floor, and for preventing the collapse of the formation from which the gas hydrate has been extracted.
Methane hydrate is deposited in underground sedimentary layers near the pole regions, hundreds to thousands meters below sea level, as a crystalline structure of methane entrapped or engaged in an expanded lattice of water, and it is regarded as a valuable resource. In order to recover the methane gas from a methane hydrate, it is necessary to change the temperature, the pressure and the balance of salt concentration of the hydrate material.
Several methods have been proposed.
(1) Heat-stimulation method (Hot water or a hot vapor is pumped into a hydrate, which it gasifies.)
(2) Depressurization method (The pressure of the gas in a hydrate is reduced.)
(3) Salt-concentration method (Salt water is pumped into a hydrate so as to promote the gasification thereof.)
(4) Chemical-injection method (Decomposition promoters such as methanol or glycol are injected into a hydrate so as to promote its gasification.)
(5) CO2-gas (or liquid CO2) replacing method (Carbon dioxide gas, which is more easily hydrated than methane is, is injected into a hydrate so as to replace the methane.)
Or, a combination of the above methods can be used.
Japanese Unexamined Patent Application No. H10-317869 proposed a high-pressure vapor-injection method (1) as mentioned above, which consists of constructing a gas-shielding wall around the hydrate stratum and then injecting high-temperature vapor to promote the decomposition of the hydrate. Japanese Unexamined Patent Application No. H9-158662 proposed the construction of a nuclear reactor at the floor of a deep sea so as to create a flow of warm surface seawater to the methane-hydrate stratum. However, because a void is produced in the sea floor stratum after the methane gas has been extracted, it is feared that the above-mentioned methods (1)-(4) can cause some deformation or collapse of the sea floor, which is fragile.
Also, in Japanese Unexamined Patent Application No. H6-71161, a carbon-dioxide-gas replacing method has been proposed. In this method, the stratum is replaced with a carbon-dioxide hydrate. However, because the CO2 gas is more easily hydrated than methane is, the injected CO2 gas is sometimes stabilized before the replacement. Therefore, CO2 gas, although favorable for the purpose of such stabilization, is economically unfavorable for the production of methane gas.
The conventional pressure-reduction method (2) also has a problem in that the possibility of continuous recovery of gas cannot be assured because it greatly depends upon the pressure of free gas, and the conventional chemical-injection method (4) has a problem in that the usage of chemicals is not economical. Furthermore, according to a survey relating to methane-hydrate strata at the sea floor, the stratum containing methane hydrate is sometimes unstable, and changes such as collapse and decomposition have occurred repeatedly in the past. From the global point of view, it is necessary on a worldwide level to prevent the dangers of troubles (geohazards) associated with landslides, large-scale sinking or rising of the sea floor, and leakage of natural gases.
The present invention has been made in view of the above-mentioned problems, and one object thereof is to provide a method of extracting a gas hydrate, whereby a gas hydrate is directly transferred to surface of the earth and the gas is recovered efficiently by controlling the decomposition of the gas, and whereby the void that results after the removal of said gas hydrate is properly filled.
Another object of the present invention is to provide an economical and safe method of extracting a gas hydrate by filling the void with industrial by-products from such industrial fields as steelmaking, power generation, and ceramic making. Another object of the present invention is to provide a method for preventing the gas-hydrate stratum from collapsing after the gas hydrate has been removed therefrom, which might cause a geohazard.
For the purpose of solving the aforementioned problems, the present invention""s method of extracting a gas hydrate is characterized such that a high-performance jet fluid is injected from a nozzle at the tip of an extraction pipe that has been inserted into a gas-hydrate stratum, and said jet fluid breaks the gas-stratum so as to form a gas-hydrate mixed fluid that is recovered on the surface of the earth, and the void that results from the removal of the gas hydrate is filled with the components of said high-performance jet fluid and a void-refilling fluid
According to the present invention, the gas hydrate, which is iced or solidified in a gas-hydrate stratum under high pressure and low temperature, is broken and is moved to the surface of the earth as a gas-hydrate mixed fluid. Therefore, the gas hydrate can be efficiently extracted from the stratum. In addition, the void resulting from the removal of the gas hydrate is filled so as to prevent the deformation of the ground after the extraction. Therefore, the extraction can be carried out safely. The gas hydrate is also safely recovered from the gas-hydrate stratum, and future geohazards, such as ground subsidence, landslides, or sinking or rising of the sea bottom, can be prevented by filling the aforementioned void.
Furthermore, a high-performance jet fluid is used for breaking the gas-hydrate stratum, so that extraction can be performed without loss of power or failure of the mechanism involved, even deeply underground or far below the surface of the sea. Also, extraction can be safely performed without adversely affecting the surrounding ground.
The extraction pipe is inserted near the bottom of the gas-hydrate stratum and is slowly retracted upwardly while rotating.
According to the present invention, the upward retraction of the injection nozzle while it is rotating can break the gas hydrate over a wide area of the stratum. Therefore, a large volume of a gas-hydrate zone can be excavated with a single well (one excavation hole), resulting in improvement of efficiency. If the extraction pipe is inserted further in the horizontal direction at the deep end (bent boring), an even wider area can be covered.
The void resulting from the removal of the gas hydrate can be filled or replaced with components of the high-performance jet fluid and the void-refilling fluid. The components are cement, chemicals, and carbon dioxide gas (CO2). The stratum can be stabilized by this method.
In addition, the gas-hydrate mixed fluid is transferred to surface of the earth as controlled by the injection pressure of the high-performance jet fluid, the speed of rotation of the injection nozzle, and the speed of retraction of the extraction pipe.
According to the present invention, the breaking or drilling volume of the gas-hydrate zone can be controlled by the rate of flow of the gas-hydrate mixed fluid, which in turn depends on the injection pressure of the high-performance jet fluid, the speed of rotation of the injection nozzle, and the speed of retraction of the extraction pipe.
The gas-hydrate mixed fluid is composed of three phases of air, including gases separated at the gas hydrate zone, water, and the solids derived from the stratum structure, and the solids are used as the components of the high-performance jet fluid and/or the void-refilling fluid.
According to the present invention, the area of the gas-hydrate zone that is broken can be controlled. Furthermore, the temperature of the high-performance jet fluid is higher than that of the gas hydrate, which serves to partially separate the gas and causes an upward flow of the gas, which is helpful in minimizing energy consumption. Sediments derived from the stratum structure in the gas-hydrate zone are separated and can be used as the components of the high-performance jet fluid and/or the void-refilling fluid.
The high-performance jet fluid is composed of air and slurry containing fine solids selected from sand and clay.
According to the present invention, the components of the high-performance jet fluid used for breaking the gas-hydrate zone can be commonly used as the void-refilling fluid that is used to fill the void in the gas-hydrate zone. Air is injected along with the high-performance jet fluid to raise the efficiency of breaking the gas-hydrate stratum.
The aforementioned fine solids are further selected from blast-furnace slag, coal ash, and killer.
According to the present invention, the use of industrial by-products can lower the cost of the void-refilling fluid and, at the same time, such use provides a means for safely disposing of industrial by-products.
Preferably the aforementioned fine solids contain at least one selected from blast-furnace slag, coal ash, and cement.
According to the present invention, the void resulting from the extraction can be filled and solidified by the use of hardening materials such as cement, blast-furnace slag, coal ash, or killer. This can prevent future landslides and ground subsidence.
The extraction pipe is a multiple-pipe structure that is composed of (a) a high-pressure pipe by which the high-performance jet fluid is conveyed to the injection nozzle at the tip, (b) a high-performance fluid duct by which the high-performance jet fluid is conveyed to the injection nozzle at the tip, and (c) a fluid-recovery pipe by which the gas-hydrate mixed fluid is transferred to surface of the earth.
According to the present invention, the multiple pipe structure can drill the gas-hydrate zone and transfer the gas-hydrate mixed fluid to the surface of the earth with one boring hole. Therefore, this is applicable to a gas-hydrate zone even under a deep-sea floor.
The water of said super high-pressure slurry is river water and spring water from the surface of the earth or seawater from near the surface of the sea.
According to the present invention, rich resources such as river water, spring water, or seawater can be favorably used, because the large temperature difference between the water and the gas-hydrate zone serves as a heat source for gas decomposition. Gas separation is further promoted by raising the temperature of the water by using sunlight or a heat source.
The extraction pipe has a control mechanism to control the pressure and speed at which said gas-hydrate mixed fluid is transferred to the surface of the earth.
According to the present invention, accidents, such as blast jet, that result from rapid gas decomposition can be prevented by controlling the pressure difference between the gas-hydrate zone and that at the surface of the earth.
The present invention""s device for extracting a gas hydrate comprises:
an extraction pipe that is composed of (a) a high-pressure pipe by which the high-performance jet fluid is conveyed to the injection nozzle at the tip, (b) a high-performance fluid duct by which the void-refilling fluid is conveyed to the injection nozzle at the tip, and (c) a fluid-recovery pipe by which the gas-hydrate mixed fluid is transferred to surface of the earth;
an extraction-pipe control unit that controls the speed of rotation and the speed of retraction of said extraction pipe;
an extracting-fluid supply unit that supplies a high-pressure fluid, a void-refilling fluid, and high-pressure air;
a pressure-control unit of the extraction pipe;
a gas-extracting device by which gases are recovered from the gas-hydrate mixed fluid;
Said device is inserted into a boring hole that has been drilled to a gas-hydrate stratum;
With the gas-hydrate extracting device of the present invention, the aforementioned gas-hydrate extracting method can be realized.
A high-performance jet fluid is injected so as to break the gas-hydrate stratum, and a void-refilling fluid is injected to fill the stratum so as to compensate for the volume of gas hydrate that has been removed.
According to the present invention, a nozzle of the high-performance jet fluid for breaking the gas-hydrate stratum and a nozzle of the void-refilling fluid are separately provided, so that both breaking and filling can be controlled. This method is realized by the multiple-pipe structure that enables a he high-performance fluid duct to be inserted into the fluid-recovery pipe.
The gas hydrate is an ice-like substance including at least methane or butane, and said gas-hydrate stratum is a zone in which said gas hydrate is buried in a state of dispersion, mass, layer, or cluster under the ground or under the sea floor.
The process of the present invention can be widely applied to the extraction of any gas hydrate other than a conventional natural-gas hydrate. Furthermore, the void of the gas-hydrate stratum that results from extraction can be filled and stabilized in both land and sea areas where troubles (geohazards) might result due to removal of the gas hydrate. Therefore, troubles (geohazards) due to deformation of the ground can be limited.